Concrete is very strong in compression, but relatively weak in tension. To compensate for this imbalance in concrete's behavior, reinforcement bars (rebar) are cast into the concrete to carry the tensile structural load. With the advent of pervious concrete, which is cement and aggregate foundation that allows water to pass, the need for a reinforcement bar that can carry the tensile structural load without reinforcement slip within the concrete or degradation is critical. Especially since the concrete may accept sea water or the snowmelt having dissolved road salt therein. The pervious concrete has been found to be most beneficial in areas that otherwise experienced flooding.
Pervious concrete pavement is known for its environmental friendly aspects, namely a way of capturing rain water and allowing the water to seep into the ground. The pervious concrete reduces storm water runoff and meets the U.S. Environmental Protection Agency (EPA) storm water regulations. The pervious concrete has a unique application by eliminating the need for retention ponds, swales, and other storm water management devices. The economic implications of allowing higher building density while meeting code compliance with regards to water retention are significant.
Pervious concrete is formed from cementious materials used to create a paste that forms a thick coating around aggregate particles. Pervious concrete contains little or no sand, creating a substantial void content. Using sufficient paste to coat and bind the aggregate particles together creates a system of highly permeable, interconnected voids that drains quickly. Typically, between 15% and 25% voids are achieved in the hardened concrete, and flow rates for water through pervious concrete are typically around 480 in./hr (0.34 cm/s, which is 5 gal/ft2/min or 200 L/m2/min), although they can be much higher. Both the low mortar content and high porosity also reduce strength compared to conventional concrete mixtures.
While pervious concrete can be used for numerous applications, its most beneficial use is in pavement that allows automobile and truck travel. Similar to non-pervious concrete, reinforcement is required in the pervious concrete to carry the tension forces. Despite steel being slightly higher thermal expansion as compared to Portland cements. Steel reinforcement bars, mesh and staples have been commonly used as the tensile reinforcement for concrete. However experience over time has shown that when steel reinforced concrete freezes or gets hot, the two materials contract and expand differently thereby assisting in the opening or micro cracks and subsequent water intrusion. Steel typically includes surface deformations to further improve that bond. Due to the strong bond, the concrete effectively transfers stresses to the steel and vice versa. Additionally, with regards to more stand density of concrete it has become popular to use short lengths of various, organic or non-organic plastic, or metal, as staples with the concrete matrix rather than reinforcement bars or mesh.
Unfortunately, while the initial bond between reinforcing steel bars and concrete is strong, the steel can deteriorate within the concrete. While cement paste helps to form a non-reactive surface film to inhibit corrosion, this passivation process is not effective if the steel is exposed to the elements. Use of steel reinforcement bars in pervious concrete immediately exposes the steel to the elements and corrosion is expected immediately. Further, the porosity of pervious concrete requires additional steel reinforcement to handle the same loads as non-pervious concrete and/or thicker layers of concrete. With regards to the recent use of various short lengths of materials as staples to reinforce pervious it is impractical as the high number of voids within the pervious is not conducive to a good mechanical strength. Rather it is subject to the ultimate interfacial peel and or sheer strength of the cement between the staple and the aggregate in the pervious concrete.
Other non-metal reinforcement bar is known in the art, including the use of fiberglass, aramid fibers such as Kevlar, Tarwon, and carbon fiber. However, fiberglass is porous and it is known to be attacked by the relatively high alkalinity levels common within Portland cements. In addition, as compared to the natural elasticity of Portland based concretes fiberglass and aramids exhibit excessive stretch to operate as perfect tensile members within the matrix. As a result the concrete may exhibit micro cracking before fiberglass and aramid reinforcements can fully “load up”. To offset these characteristics engineers must design in more of these FRP materials to prevent excessive panel or beam deflection as compared to traditional steels or carbon fiber reinforcements. Carbon fiber reinforcements however exhibit extremely short stretch and elongation to break. As a result an engineer must insure the use of enough CFRP to carry all the tensile loading of the matrix. These factors coupled with the high expense of carbon fiber make its use impractical.
More recently, reinforcement bar produced from continuous basalt fiber has been found to be superior to steel in both pervious and non-pervious concrete. The Applicant, Raw Energy Materials of Pompano Beach Fla., is a manufacture of several continuous basalt fiber reinforcements marked under the trade name RockRebar™, Rock Stirrups™, RockMesh™ and RockDNA™ that is now commonly placed within concrete, including water passing pervious concrete.